This invention is concerned with a workpiece-gripping device for tool machines. As well known, mechanical parts are generally manufactured starting from a steel or aluminum blank on which a tool machine having an operating head movable along one or more directions performs a number of machining operations such as boring, facing, drilling, tapping, and the like. Highly automated tool machines can automatically pick up a required tool from a tool magazine located beside the operating head of the machine. Loading and unloading of the workpiece to and from the worktable can be performed manually or, for a a higher productivity, by automated loading/unloading devices, such as disclosed in U.S. Pat. No. 4,716,647. There, the operating head picks the workpiece by means of a gripping device that is automatically mounted on the chuck, similarly to a regular tool. The head places the workpiece on the worktable, and the gripping device is then returned to the tool magazine, while a tool is mounted on the chuck. After the machining schedule is completed, the tool is returned to the tool magazine and the gripping device is again picked up, so that the operating head can remove the newly machined workpiece from the worktable and pick up a fresh one.
Known gripping devices such as disclosed in the above prior document are provided with pincers controlled by hydraulic cylinders driven by an external hydraulic circuit. However, there are drawbacks in hydraulically controlled pincers, notably that a supply circuit for high-pressure fluid has to be provided, with attendant increase of the costs of manufacturing, operating and maintaining the tool machine. Moreover, the very existence of the hydraulic circuit involves unavoidable long-term leakages due to wear of the controlling members, as well as leakages in the fluid-supply line. This circumstance leads to pollution of the working area and to possible malfunction.
Further, due to the intrinsic nature of hydraulic control systems, the delay from the control command and the completion of the actual mechanical event is never known accurately. Such a delay may vary considerably among different actions, and may change in the course of time due to line leakages, cylinder wear, and the like. Consequently, before displacing the head it is necessary to allow a certain time delay that is longer than the average foreseeable reaction time, with attendant lengthening of the work schedule.